Speed controlling an electric machine of a hybrid electric vehicle

ABSTRACT

A hybrid powertrain includes an engine having a crankshaft, and an electric motor having a rotor selectively coupled to the crankshaft via a disconnect clutch. The powertrain further includes a transmission having a torque converter that has an impeller fixed to the rotor. A controller is configured to, in response to the engine starting, generate a torque command for the motor that defines a magnitude that is based on a difference between a target impeller speed and a measured impeller speed.

TECHNICAL FIELD

The present disclosure relates to hybrid-electric vehicles and morespecifically to controlling an electric machine using speed controlduring certain operating conditions.

BACKGROUND

A hybrid-electric powertrain includes an engine and an electric machine.The torque (or power) produced by the engine and/or the electric machinecan be transferred through a transmission to the driven wheels to propelthe vehicle. A traction battery supplies energy to the electric machine.

SUMMARY

According to one embodiment, a hybrid powertrain includes an enginehaving a crankshaft, and an electric motor having a rotor selectivelycoupled to the crankshaft via a disconnect clutch. The powertrainfurther includes a transmission having a torque converter that has animpeller fixed to the rotor. A controller is configured to, in responseto the engine starting, generate a torque command for the motor thatdefines a magnitude that is based on a difference between a targetimpeller speed and a measured impeller speed.

According to another embodiment, a vehicle includes an engine having acrankshaft and a transmission. The transmission includes a torqueconverter having an impeller, and a turbine fixed to a turbine shaftthat is driveably connected to driven wheels of the vehicle. The torqueconverter further has a bypass clutch configured to selectively lock theimpeller and turbine relative to each other. An electric machine has arotor selectively coupled to the crankshaft via a disconnect clutch. Therotor is fixed to the impeller. A speed sensor is disposed within thetransmission and is configured to output a speed signal indicating ameasured impeller speed. At least one controller of the vehicle isconfigured to, in response to a change in torque split between theengine and the electric machine, and the bypass clutch being open orslipping, generate a torque command for the electric machine thatincludes a feedforward component, and a feedback component that is basedon an error between a target impeller speed and the measured impellerspeed.

According to yet another embodiment, a method of controlling an electricmachine of a hybrid powertrain is disclosed. The powertrain includes anengine, a transmission, and a torque converter. The torque converterincludes a turbine, an impeller fixed to the electric machine, and abypass clutch. The method includes generating a command to start theengine. The method further includes, in response to the command to startthe engine and the bypass clutch being open or slipping, generating aspeed-control torque command for the electric machine that defines amagnitude that is based on a difference between a target impeller speedand a measured impeller speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an example hybrid-electric vehicle.

FIG. 2 shows variations of the powertrain parameters during an enginestart in which the electric machine is controlled in torque control.

FIGS. 3A to 3C show a flow chart of a control strategy for starting theengine.

FIG. 4 is a control diagram illustrating a speed-control algorithm forcontrolling the electric machine.

FIG. 5 shows variations of the powertrain parameters during an enginestart in which the electric machine is controlled in speed control.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the embodiments. Asthose of ordinary skill in the art will understand, various featuresillustrated and described with reference to any one of the figures canbe combined with features illustrated in one or more other figures toproduce embodiments that are not explicitly illustrated or described.The combinations of features illustrated provide representativeembodiments for typical applications. Various combinations andmodifications of the features consistent with the teachings of thisdisclosure, however, could be desired for particular applications orimplementations.

Referring to FIG. 1, a schematic diagram of a hybrid-electric vehicle(HEV) 10 is illustrated according to an embodiment of the presentdisclosure. FIG. 1 illustrates representative relationships among thecomponents. Physical placement and orientation of the components withinthe vehicle may vary. The HEV 10 includes a powertrain 12 having anengine 14 that drives a transmission 16, which may be referred to as amodular-hybrid transmission (MHT). As will be described in furtherdetail below, a transmission 16 includes an electric machine such as anelectric motor/generator (M/G) 18, an associated traction battery 20, atorque converter 22, and a multiple step-ratio automatic transmission,or gearbox 24. The M/G may also be referred to as the motor 18.

The engine 14 and the M/G 18 are both drive sources for the HEV 10. Theengine 14 generally represents a power source that may include aninternal-combustion engine such as a gasoline, diesel, or natural gaspowered engine, or a fuel cell. The engine 14 generates an engine powerand corresponding engine torque that is supplied to the M/G 18 when adisconnect clutch (KO clutch) 26 between the engine 14 and the M/G 18 isat least partially engaged. The M/G 18 may be implemented by any one ofa plurality of types of electric machines. For example, M/G 18 may be apermanent-magnet-synchronous motor. Power electronics 56 conditiondirect current (DC) provided by the battery 20 to the requirements ofthe M/G 18, as will be described below. For example, power electronicsmay provide three-phase alternating current (AC) to the M/G 18.

When the disconnect clutch 26 is at least partially engaged, power flowsfrom the engine 14 to the M/G 18. Power flow from the M/G 18 to theengine 14 is also possible. For example, the disconnect clutch 26 may beengaged and M/G 18 may operate as a generator to convert rotationalenergy provided by a crankshaft 28 and M/G shaft 30 into electricalenergy to be stored in the battery 20. The disconnect clutch 26 can alsobe disengaged to isolate the engine 14 from the remainder of thepowertrain 12 such that the M/G 18 can act as the sole drive source forthe HEV 10. The shaft 30 extends through the M/G 18. The rotor 19 of theM/G 18 is fixed on the shaft 30, whereas the engine 14 is selectivelydriveably connected to the shaft 30 only when the disconnect clutch 26is at least partially engaged.

A separate starter motor 31 can be selectively engaged with the engine14 to rotate the engine to allow combustion to begin. The starter motor31 may be powered by a 12-volt system of the vehicle. Once the engine isstarted, the starter motor 31 can be disengaged from the engine via, forexample, a solenoid that engages/disengages a pinion gear with the ringgear on the flywheel (not shown). In one embodiment, the engine 14 isstarted by the starter motor 31 while the disconnect clutch 26 is open,keeping the engine disconnected with the M/G 18. Once the engine hasstarted and is brought up to speed with the M/G 18, the disconnectclutch 26 can couple the engine to the M/G to allow the engine toprovide drive torque.

In another embodiment, the starter motor 31 is not provided and,instead, the engine 14 is started by the M/G 18. To do so, thedisconnect clutch 26 partially engages to transfer torque from the M/G18 to the engine 14. The M/G 18 may be required to ramp up in torque tofulfill driver demands while also starting the engine 14. The disconnectclutch 26 can then be fully engaged once the engine speed is brought upto the speed of the M/G.

The M/G 18 is driveably connected to the torque converter 22 via theshaft 30. For example, the torque-converter housing may be fastened tothe shaft 30. The torque converter 22 is therefore driveably connectedto the engine 14 when the disconnect clutch 26 is at least partiallyengaged. Two components are driveably connected if they are connected bya power flow path that constrains their rotational speeds to be directlyproportional. The torque converter 22 includes an impeller 35 fixed tothe torque-converter housing (and consequently, fixed to the rotor 19)and a turbine 37 fixed to a transmission input shaft 32 that isdriveably connected to the driven wheels 42. The torque converter 22provides a hydraulic coupling between the shaft 30 and the transmissioninput shaft 32. The torque converter 22 transmits power from theimpeller 35 to the turbine 37 when the impeller rotates faster than theturbine. The magnitude of the turbine torque and impeller torquegenerally depend upon the relative speeds. When the ratio of impellerspeed to turbine speed is sufficiently high, the turbine torque is amultiple of the impeller torque. A torque converter bypass clutch 34 maybe provided to, when engaged, frictionally or mechanically couple theimpeller and the turbine of the torque converter 22, permitting moreefficient power transfer. The torque converter bypass clutch 34 may beoperated as a launch clutch to provide smooth vehicle launch.Alternatively, or in combination, a launch clutch similar to disconnectclutch 26 may be provided between the M/G 18 and gearbox 24 forapplications that do not include a torque converter 22 or a torqueconverter bypass clutch 34. In some applications, the disconnect clutch26 is generally referred to as an upstream clutch and launch clutch 34(which may be a torque converter bypass clutch) is generally referred toas a downstream clutch.

The gearbox 24 may include gear sets (not shown) that are selectivelyplaced in different gear ratios by selective engagement of frictionelements such as clutches and brakes (not shown) to establish thedesired multiple discrete or step drive ratios. The friction elementsare controllable through a shift schedule that connects and disconnectscertain elements of the gear sets to control the ratio between atransmission output shaft 38 and the transmission input shaft 32. Thegearbox 24 is automatically shifted from one ratio to another based onvarious vehicle and ambient operating conditions by an associatedcontroller, such as a powertrain-control unit (PCU) 50. The gearbox 24then provides powertrain output torque to output shaft 38. The outputshaft 38 may be connected to a driveline 37 (e.g., a driveshaft anduniversal joints) that connects the output shaft 38 to the differential40.

It should be understood that the hydraulically controlled gearbox 24used with a torque converter 22 is but one example of a gearbox ortransmission arrangement; any multiple-ratio gearbox that accepts inputtorque(s) from an engine and/or a motor and then provides torque to anoutput shaft at the different ratios is acceptable for use withembodiments of the present disclosure. For example, gearbox 24 may beimplemented by an automated mechanical (or manual) transmission (AMT)that includes one or more servo motors to translate/rotate shift forksalong a shift rail to select a desired gear ratio. As generallyunderstood by those of ordinary skill in the art, an AMT may be used inapplications with higher torque requirements, for example.

As shown in the representative embodiment of FIG. 1, the output shaft 38may be connected to a driveline 37 that connects the output shaft 38 tothe differential 40. The differential 40 drives a pair of wheels 42 viarespective axles 44 connected to the differential 40. The differentialtransmits approximately equal torque to each wheel 42 while permittingslight speed differences such as when the vehicle turns a corner.Different types of differentials or similar devices may be used todistribute torque from the powertrain to one or more wheels. In someapplications, torque distribution may vary depending on the particularoperating mode or condition, for example.

While illustrated as one controller, the controller 50 may be part of alarger control system and may be controlled by various other controllersthroughout the vehicle 10, such as a vehicle-system controller (VSC) anda high-voltage battery controller (BECM). It is to be understood thatthe powertrain-control unit 50 and one or more other controllers cancollectively be referred to as a “controller” that controls variousactuators in response to signals from various sensors to controlfunctions such as starting/stopping engine 14, operating M/G 18 toprovide wheel torque or charge the battery 20, select or scheduletransmission shifts, etc. For example, the M/G 18 may include a speedsensor 45 configured to output a signal to the controller 50 that isindicative of a speed of the M/G. The controller 50 may include amicroprocessor or central processing unit (CPU) in communication withvarious types of computer readable storage devices or media. Computerreadable storage devices or media may include volatile and nonvolatilestorage in read-only memory (ROM), random-access memory (RAM), andkeep-alive memory (KAM), for example. KAM is a persistent ornon-volatile memory that may be used to store various operatingvariables while the CPU is powered down. Computer-readable storagedevices or media may be implemented using any of a number of knownmemory devices such as PROMs (programmable read-only memory), EPROMs(electrically PROM), EEPROMs (electrically erasable PROM), flash memory,or any other electric, magnetic, optical, or combination memory devicescapable of storing data, some of which represent executableinstructions, used by the controller in controlling the engine, tractionbattery, transmission, or other vehicle systems.

The controller communicates with various engine/vehicle sensors andactuators via an input/output (I/O) interface that may be implemented asa single integrated interface that provides various raw data or signalconditioning, processing, and/or conversion, short-circuit protection,and the like. Alternatively, one or more dedicated hardware or firmwarechips may be used to condition and process particular signals beforebeing supplied to the CPU. As generally illustrated in therepresentative embodiment of FIG. 1, the controller 50 may communicatesignals to and/or from the engine 14, disconnect clutch 26, M/G 18,launch clutch 34, transmission gearbox 24, and power electronics 56.Although not explicitly illustrated, those of ordinary skill in the artwill recognize various functions or components that may be controlled bycontroller 50 within each of the subsystems identified above.Representative examples of parameters, systems, and/or components thatmay be directly or indirectly actuated using control logic executed bythe controller include fuel injection timing, rate, and duration,throttle valve position, spark plug ignition timing (for spark-ignitionengines), intake/exhaust valve timing and duration, front-end accessorydrive (FEAD) components such as an alternator, air-conditioningcompressor, battery charging, regenerative braking, M/G operation,clutch pressures for disconnect clutch 26, launch clutch 34, andtransmission gearbox 24, and the like. Sensors communicating inputthrough the I/O interface may be used to indicate turbocharger boostpressure (if applicable), crankshaft position (PIP), engine rotationalspeed (RPM), wheel speeds (WS1, WS2), vehicle speed (VSS), coolanttemperature (ECT), intake-manifold pressure (MAP), accelerator-pedalposition (PPS), ignition-switch position (IGN), throttle-valve position(TP), air temperature (TMP), exhaust-gas oxygen (EGO) or other exhaustgas component concentration or presence, intake-air flow (MAF),transmission gear, ratio, or mode, transmission-oil temperature (TOT),transmission-turbine speed (TS), torque converter bypass clutch 34status (TCC), deceleration or shift mode (MDE), for example.

Control logic or functions performed by controller 50 may be representedby flow charts or similar diagrams in one or more figures. These figuresprovide representative control strategies and/or logic that may beimplemented using one or more processing strategies such asevent-driven, interrupt-driven, multi-tasking, multi-threading, and thelike. As such, various steps or functions illustrated may be performedin the sequence illustrated, in parallel, or in some cases omitted.Although not always explicitly illustrated, one of ordinary skill in theart will recognize that one or more of the illustrated steps orfunctions may be repeatedly performed depending upon the particularprocessing strategy being used. Similarly, the order of processing isnot necessarily required to achieve the features and advantagesdescribed herein, but is provided for ease of illustration anddescription. The control logic may be implemented primarily in softwareexecuted by a microprocessor-based vehicle, engine, and/or powertraincontroller, such as controller 50. Of course, the control logic may beimplemented in software, hardware, or a combination of software andhardware in one or more controllers depending upon the particularapplication. When implemented in software, the control logic may beprovided in one or more computer-readable storage devices or mediahaving stored data representing code or instructions executed by acomputer to control the vehicle or its subsystems. The computer-readablestorage devices or media may include one or more of a number of knownphysical devices which utilize electric, magnetic, and/or opticalstorage to keep executable instructions and associated calibrationinformation, operating variables, and the like.

An accelerator pedal 52 is used by the driver of the vehicle to providea demanded torque, power, or drive command to propel the vehicle. Thepedal 52 may include a pedal-position sensor. In general, depressing andreleasing the pedal 52 causes the pedal sensor to generate anaccelerator-pedal-position signal that may be interpreted by thecontroller 50 as a demand for increased power or decreased power,respectively. Based at least upon input from the pedal, the controller50 commands torque from the engine 14 and/or the M/G 18. The controller50 also controls the timing of the gear shifts within the gearbox 24, aswell as engagement or disengagement of the disconnect clutch 26 and thetorque converter bypass clutch 34. Like the disconnect clutch 26, thebypass clutch 34 can be modulated across a range between the engaged anddisengaged positions. This produces a variable slip in the torqueconverter 22 in addition to the variable slip produced by thehydrodynamic coupling between the impeller and the turbine.Alternatively, the bypass clutch 34 may be operated as locked or openwithout using a modulated operating mode depending on the particularapplication.

To drive the vehicle with the engine 14, the disconnect clutch 26 is atleast partially engaged to transfer at least a portion of the enginetorque through the disconnect clutch 26 to the M/G 18, and then from theM/G 18 through the torque converter 22 and gearbox 24. When the engine14 alone provides the torque necessary to propel the vehicle, thisoperation mode may be referred to as the “engine mode,” “engine-onlymode,” or “mechanical mode.”

The M/G 18 may assist the engine 14 by providing additional power toturn the shaft 30. This operation mode may be referred to as “hybridmode,” “engine-motor mode,” or “electric-assist mode.”

To drive the vehicle with the M/G 18 as the sole power source, the powerflow remains the same except the disconnect clutch 26 isolates theengine 14 from the remainder of the powertrain 12. Combustion in theengine 14 may be disabled or otherwise OFF during this time to conservefuel. The traction battery 20 transmits stored electrical energy throughwiring 54 to power electronics 56 that may include an inverter and aDC/DC converter, for example. The power electronics 56 convert DCvoltage from the battery 20 into AC voltage to be used by the M/G 18.The controller 50 commands the power electronics 56 to convert voltagefrom the battery 20 to an AC voltage provided to the M/G 18 to providepositive (e.g. drive) or negative (e.g. regenerative) torque to theshaft 30. This operation mode may be referred to as an “electric onlymode,” “EV (electric vehicle) mode,” or “motor mode.”

In any mode of operation, the M/G 18 may act as a motor and provide adriving force for the powertrain 12. Alternatively, the M/G 18 may actas a generator and convert kinetic energy from the powertrain 12 intoelectric energy to be stored in the battery 20. The M/G 18 may act as agenerator while the engine 14 is providing propulsion power for thevehicle 10, for example. The M/G 18 may additionally act as a generatorduring times of regenerative braking in which rotational energy fromspinning wheels 42 is transferred back through the gearbox 24 and isconverted into electrical energy for storage in the battery 20.

It should be understood that the schematic illustrated in FIG. 1 ismerely an example and is not intended to be limiting. Otherconfigurations are contemplated that utilize selective engagement ofboth an engine and a motor to transmit through the transmission. Forexample, the M/G 18 may be offset from the crankshaft 28, and/or the M/G18 may be provided between the torque converter 22 and the gearbox 24.Other configurations are contemplated without deviating from the scopeof the present disclosure.

The vehicle-control system (which includes the controller 50) determinesa driver-demanded torque based on signals from a pedal-position sensorassociated with the accelerator pedal 52. This torque may be deliveredby placing the powerplants (e.g., engine and motor) in torque control.In torque control, the controller determines a torque split between theengine and the motor and commands that torque from each of thepowerplants.

Referring to FIG. 2, an example engine start is shown. In this example,the engine and the motor are in torque control and the engine is startedusing the disconnect clutch (as opposed to an auxiliary starter motor).In response to an engine start request, the controller estimates theclutch capacity 63 and determines a commanded motor torque 70. Duringengine start, the commanded motor torque 70 is equal to thedriver-demanded torque minus the disconnect clutch torque. Thedisconnect clutch torque is the disconnect clutch capacity with anegative sign when the disconnect clutch is slipping and the enginespeed is lower than the motor speed. The disconnect clutch torque is thedisconnect clutch capacity with a positive sign when the disconnectclutch is slipping and the engine speed is higher than the motor speed.At time T0, the disconnect clutch capacity 62 begins to rise and theclutch begins to close. At time T1, the crankshaft of the engine beginsto rotate as shown by the engine speed trace 64. At time T3, the enginebegins to produce torque as shown by the engine torque trace 66. Theengine torque rapidly increases from time T3 to time T4, when the engineis started.

Using torque control, the engine and the motor are controlled to matchspeeds so that the disconnect clutch can be fully closed to lock theengine and motor. Ideally, the engine and motor would be locked at point68. But, the engine torque is larger than the actual disconnect clutchcapacity 61. The clutch locks when the motor and engine speeds match andthe disconnect clutch capacity exceeds the torque produced by theengine. As such, locking of the engine and the motor is delayed to point76, where engine torque 66 is less than the clutch capacity 62.

The estimated disconnect clutch capacity 63 is not a perfect predictionof the actual clutch capacity 62. Because of this inaccuracy, the systemis not able to perfectly deliver the desired impeller torque. The motortorque that is required to perfectly deliver the desired torque isrepresented with line 60. The difference between the ideal motor torque60 and the actual motor torque 70 is shown by trace 72. This representsthe torque disturbance created when the controller fails to correctlycompensate for the disconnect clutch torque. The error 72 causes themeasured motor speed 74 to dip below the desired motor speed 78 duringengine start. This decrease in motor speed reduces torque transmissionthrough the torque converter and reduces vehicle acceleration. It is tobe noted that the desired motor speed 78 is not a target or commandedspeed as there is no target motor speed in torque control. The error 72further causes the motor speed 74 to be higher than the desired speed 78after the engine and motor are locked. This decrease in motor speedincreases torque transmission through the torque converter and increasevehicle acceleration. These changes in acceleration produce powertraindisturbances that are perceivable by the driver.

In vehicles with multiple powerplants, such as vehicle 10, it isimportant that each powerplant accurately produces the demanded torque.Inaccuracies in the torque can lead to vehicle speed increasing ordecreasing without the driver requesting it. Accurately controlling thepowerplants using torque control is particularly difficult duringtransition events where the torque split between the actuators changes,such as engine start because it is difficult to accurately estimate theinstantaneous capacity of the disconnect clutch. During engine start, itmay be advantageous to utilize speed control of at least one of theactuators to reduce torque delivery errors. For example, the motor 18may be placed in speed control during engine start. In speed control,the controller may set a target motor speed and measure the speed of themotor. The controller may compare these speeds and output a torquerequest to the motor based on an error between these speeds.

Speed control provides an inherent robustness to torque delivery errors.In the following example, a torque converter model is used to generate atarget motor speed, which allows M/G 18 to be controlled using speedcontrol. Speed control of the M/G can be utilized during any conditionwhere the torque converter clutch is not fully locked (i.e., open orslipping). When the driver applies the accelerator pedal, the vehiclecontrol system determines a driver-demanded torque. As long as thetorque-converter capacity is less than this driver-demanded torque,there will be slip across the torque converter. The amount of slip, andthus the desired impeller speed, can be predicted using a model of thetorque converter. Because achieving the speed target is equivalent toachieving driver-demanded torque, the M/G can be placed in speed controland track this target. This provides robustness against torque deliveryerrors. An example algorithm for speed controlling the M/G 18 will bedescribed below in more detail.

Referring to FIGS. 3A to 3C, a flow chart 100 of an algorithm forstarting the engine 14 is shown. The method is implemented usingsoftware code contained within the vehicle control module (e.g.controller 50), according to one or more embodiments. In otherembodiments, the method 100 is implemented in other vehicle controllers,or distributed amongst multiple vehicle controllers.

The method of controlling engine start in the hybrid electric vehiclemay be implemented through a computer algorithm, machine-executablecode, or software instructions programmed into a suitable programmablelogic device(s) of the vehicle, such as the vehicle control module, thehybrid control module, other controller in communication with vehiclecomputing system, or a combination thereof. Although the various stepsshown in the flowchart diagram 100 appear to occur in a chronologicalsequence, at least some of the steps may occur in a different order, andsome steps may be performed concurrently or not at all.

At operation 102 the controller 50 determines if the engine 14 isconnected to the motor 18. If yes, control loops back to the start. Ifno, the controller receives an accelerator pedal-position signal atoperation 104 from a sensor associated with the pedal 52. Using thepedal-position signal, the controller determines a driver-demandedtorque at operation 106. At operation 108, the controller determines thespeed of the turbine 37, which may be derived from the speed of thevehicle.

At operation 110 the controller determines the impeller speed requiredto meet driver-demanded torque. The impeller speed may be calculatedusing equation 1:

τ_(turbine) =K(ω_(I))²+τ_(bypass clutch)  Eq. (1)

where, τ_(turbine) is torque at the turbine, τ_(bypass) is torque on thetorque converter bypass clutch, ω_(I) is the impeller speed and K is af(turbine speed and impeller speed).

Since the desired turbine torque (equal to driver-demanded torque), thetorque converter bypass clutch capacity, and the turbine speed areknown, the controller can determine an impeller speed that provides thedriver-demanded torque using equation 1.

At operation 112, the controller determines if an engine start is inprogress. If no, control passes to operation 114 and the controllerdetermines if an engine start is being requested. If no, control loopsback to the start. If yes, control passes to operation 120. At operation120 the controller determines which cranking device is going to be usedto start the engine. In the illustrated vehicle 10, the engine 14 can bestarted with either the dedicated starter 31 or by the motor 18 inconjunction with the disconnect clutch 26. If it is determined atoperation 122 that the starter 31 is being used, control passes tooperation 124, and if the disconnect clutch is being used, controlpasses to operation 128. At operation 124 voltage is supplied to thestarter 31 to crank the engine 14. In operation 126 the controllercommands the disconnect clutch to stroke in order to connect to thecrankshaft 28 and the shaft 30. If the disconnect clutch is being usedto start the engine, the controller determines the desireddisconnect-clutch capacity for cranking the engine at operation 128. Atoperation 130 the controller commands the disconnect clutch to apressure calculated to supply the capacity determined in operation 128.In some embodiments, the controller may measure the disconnect clutchpressure at operation 132. This step is optional. At operation 134 thecontroller estimates the capacity of the disconnect clutch 26. Inoperation 136 the speeds of the motor and the engine are determined. Oneor both of these speeds may be directly measured by sensors or may beinferred from other inputs. At operation 138, the controller estimatesthe motor torque and the engine torque.

At operation 140, the controller determines if the engine-cranking phaseis complete. The engine-cranking phase is defined between the start ofengine cranking and the first combustion of the engine. The controllermay determine the end of the engine-cranking phase by measuring theengine speed and comparing it to a minimum starting speed. If the enginespeed exceeds that speed and fuel has been injected and combusted, theengine-cranking phase is complete.

If the engine cranking phase is not complete control passes to operation142, and the controller determines the motor torque required to deliverthe driver-demanded torque while compensating for the engine start.

Accurately estimating the instantaneous disconnect-clutch capacity isdifficult and may lead to torque delivery errors. To reduce these torqueerrors, the commanded motor torque can be adjusted using a feedback loopthat utilizes speed control. Equation 1 may be used to speed control themotor when the torque-converter lockup clutch 134 is open or slipping.At operation 144, the controller determines if the torque-converterlockup clutch is open or slipping. If the lockup clutch is locked,control passes to operation 146 and the motor 18 is placed in torquecontrol because speed control is not available. If the clutch is open orslipping, control passes to operation 148 and the motor 18 is placed inspeed control.

FIG. 4 illustrates controls for operating the motor 18 in speed control.The controller receives an accelerator pedal-position signal and animpeller-speed signal at box 200, and based on those inputs, determinesa driver-demanded torque. The driver-demanded torque is fed into afeedforward controller 202. The feedforward controller 202 also receivesa capacity estimate of the disconnect clutch. Based on those inputs, thefeedforward controller 202 outputs a feedforward torque. Thedriver-demanded torque is also feed into box 204. Box 204 also receivesthe speed of the turbine and an estimated capacity of thetorque-converter clutch. Based on these inputs, the controllerdetermines the required impeller speed to meet the driver-demandedtorque using equation 1. Box 204 outputs a target impeller-speed signalto comparison box 206. Box 206 receives a measured impeller-speedsignal, and compares the target impeller speed to the measured impellerspeed to determine an error, which is fed to the feedback controller208. The feedback controller 208 converts the speed error into afeedback torque and outputs the feedback torque to the impeller-speedcontroller 210. The impeller-speed controller 210 combines thefeedforward torque and the feedback torque and outputs a speed-controltorque command to the motor 18 that is based on a difference between atarget impeller speed and the measured impeller speed. Speed controlprovides a robustness to torque delivery errors. Speed control is alsorobust to errors in the torque-converter model because it ismonotonic—an increasing driver-demanded torque request will alwaysresult in an increasing target impeller speed. If the impeller speedtarget fails deliver the driver's expectations, the driver will simplymodulate the accelerator pedal until the vehicle produces the desiredresponse.

Referring back to operation 112, if the engine start is in progresscontrol passes operation 116 and the controller determines if a lockupphase of the disconnect clutch 26 is active. The lockup phase occurswhen the engine and motor speeds are within a predefined threshold ofeach other and the disconnect clutch begins to lock the crankshaft 28and the shaft 30. If no at operation 116, control passes to operation118 and determines if a run-up phase of the engine is active. The run-upphase occurs between the engine-cranking phase and the lockup phase. Ifno at operation 118, control passes to operation 120. If yes atoperation 118, control passes operation 150.

At operation 150 the controller determines the desired disconnect-clutchcapacity for engine run up. At operation 152 the controller commands thedisconnect clutch to a pressure calculated to supply the capacitydetermined in operation 150. In some embodiments, the controller maymeasure the disconnect clutch pressure at operation 154. This step isoptional. In operation 156 the controller estimates the capacity of thedisconnect clutch 26. At operations 158 and 160 the speeds and torquesof the motor and the engine are determined. At operation 162 thecontroller determines if the disconnect clutch is ready to be locked.The disconnect clutch is ready to be locked when engine speed isapproximates motor speed, and the engine acceleration approximates themotor acceleration.

If the disconnect clutch is not ready to be locked, control passes tooperation 164. At operation 164 the controller determines the motortorque required to deliver the driver-demanded torque while compensatingfor disconnect clutch capacity. At operation 166, the controllerdetermines if the torque-converter lockup clutch is open or slipping. Ifthe lockup clutch is locked, control passes to operation 168 and themotor 18 is placed in torque-control mode. If the clutch is open orslipping, control passes operation 170 and the motor 18 is placed in aspeed-control as described in FIG. 4 for example.

If the disconnect clutch is ready to be locked at operation 162, controlpasses operation 172, and the controller determines a desired capacityto lockup the disconnect clutch during engine run up. At operation 174the controller commands the disconnect clutch to a pressure calculatedto supply the capacity determined in operation 172. In some embodiments,the controller may measure the disconnect clutch pressure at operation176. In operation 178 the controller estimates the capacity of thedisconnect clutch 26. At operations 180 and 182 the speeds and torquesof the motor and the engine are determined. At operation 184 thecontroller determines the motor torque required to deliver thedriver-demanded torque while compensating for disconnect clutchcapacity.

At operation 186, the controller determines if the engine is connectedand the starting process complete. If yes, control passes to operation188 and the motor is placed in torque control. If no, control passesoperation 190, and the controller determines if the torque converterlockup clutch is open or slipping. If the lockup clutch is locked,control passes to operation 192 and the motor 18 is placed intorque-control mode. If the clutch is open or slipping, control passesto operation 194 and the motor 18 is placed in a speed-control mode.

Referring to FIG. 5, an example engine start is shown. In this example,the engine is in torque control, the motor is in speed control, and theengine is being started using the disconnect clutch. It is to beunderstood, however, that the teachings of this disclosure are equallyapplicable to engine starts using the auxiliary starter 31. The exampleengine start of FIG. 5 utilizes the algorithm of FIGS. 3A-3C. ComparingFIGS. 2 (torque control) and 5 (speed control), the advantages of speedcontrolling the M/G 18 become apparent. The measured motor speed 220follows the target motor speed 222 more closely than the measured motorspeed 74 follows the desired motor speed 78 in FIG. 2. This producessmoother vehicle acceleration and less jerkiness. Using speed control(as described in FIG. 4), the controller may command a feedforwardtorque command 224 and a feedback torque command 226. The feedbacktorque command is based on a difference between the target impellerspeed and the measured impeller speed. By adjusting the motor torqueaccording to impeller speed, the measured motor speed 220 can be made tofollow the target motor speed 222 more closely than using torquecontrol.

The processes, methods, or algorithms disclosed herein can bedeliverable to/implemented by a processing device, controller, orcomputer, which can include any existing programmable electronic controlunit or dedicated electronic control unit. Similarly, the processes,methods, or algorithms can be stored as data and instructions executableby a controller or computer in many forms including, but not limited to,information permanently stored on non-writable storage media such as ROMdevices and information alterably stored on writeable storage media suchas floppy disks, magnetic tapes, CDs, RAM devices, and other magneticand optical media. The processes, methods, or algorithms can also beimplemented in a software executable object. Alternatively, theprocesses, methods, or algorithms can be embodied in whole or in partusing suitable hardware components, such as Application SpecificIntegrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs),state machines, controllers or other hardware components or devices, ora combination of hardware, software and firmware components.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. These attributes caninclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, to the extentany embodiments are described as less desirable than other embodimentsor prior art implementations with respect to one or morecharacteristics, these embodiments are not outside the scope of thedisclosure and can be desirable for particular applications.

What is claimed is:
 1. A hybrid powertrain comprising: an engine havinga crankshaft; an electric motor including a rotor selectively coupled tothe crankshaft via a disconnect clutch; a transmission including atorque converter having an impeller fixed to the rotor; and a controllerconfigured to, in response to the engine starting, generate a torquecommand for the motor that defines a magnitude that is based on adifference between a target impeller speed and a measured impellerspeed.
 2. The hybrid powertrain of claim 1, wherein the torque converterfurther includes a bypass clutch configured to selectively lock theimpeller and turbine relative to each other.
 3. The hybrid powertrain ofclaim 2, wherein the controller is further configured to generate thetorque command in response to the bypass clutch being open or slipping.4. The hybrid powertrain of claim 1, wherein the torque command furtherincludes a feedforward component, and a feedback component that is basedon the difference between the target impeller speed and the measuredimpeller speed.
 5. The hybrid powertrain of claim 4, wherein thefeedforward component is based on a capacity of a torque converterbypass clutch.
 6. The hybrid powertrain of claim 1, wherein themagnitude increases in response to the difference between the targetimpeller speed and the measured impeller speed increasing.
 7. The hybridpowertrain of claim 6, wherein the magnitude decreases in response tothe difference between the commanded impeller speed and the measuredimpeller speed decreasing.
 8. The hybrid powertrain of claim 1, whereinthe controller is further configured to command starting of the engine.9. The hybrid powertrain of claim 1 further comprising a speed sensordisposed within the electric motor and configured to output a speedsignal indicating a measured impeller speed.
 10. A vehicle comprising:an engine including a crankshaft; a transmission including a torqueconverter having an impeller, and a turbine fixed to a turbine shaftthat is driveably connected to driven wheels of the vehicle, wherein thetorque converter further includes a bypass clutch configured toselectively lock the impeller and turbine relative to each other; anelectric machine including a rotor selectively coupled to the crankshaftvia a disconnect clutch and fixed to the impeller; a speed sensordisposed within the transmission and configured to output a speed signalindicating a measured impeller speed; and at least one controllerconfigured to, in response to a change in torque split between theengine and the electric machine, and the bypass clutch being open orslipping, generate a torque command for the electric machine thatincludes a feedforward component, and a feedback component that is basedon an error between a target impeller speed and the measured impellerspeed.
 11. The vehicle of claim 10, wherein the change in torque splitincludes starting of the engine.
 12. The vehicle of claim 10, whereinthe feedforward component is based on a capacity of the bypass clutch.13. The vehicle of claim 10, wherein the feedforward component is basedon a capacity of the disconnect clutch.
 14. The vehicle of claim 12,wherein the feedforward component is based on a pedal position of anaccelerator pedal of the vehicle.
 15. The vehicle of claim 11, whereinthe controller is further programmed to command closing of thedisconnect clutch in response to a request to start the engine.
 16. Thevehicle of claim 10, wherein a magnitude of the feedback componentincreases in response to the error increasing.
 17. The vehicle of claim10, wherein the target impeller speed is based on a measured speed ofthe turbine.
 18. A method of controlling an electric machine of a hybridpowertrain that includes an engine, a transmission, and a torqueconverter having a turbine, an impeller fixed to the electric machine,and a bypass clutch, the method comprising: generating a command tostart the engine; and in response to the command to start the engine andthe bypass clutch being open or slipping, generating a speed-controltorque command for the electric machine that defines a magnitude that isbased on a difference between a target impeller speed and a measuredimpeller speed.
 19. The method of claim 18 further comprising, inresponse to completion of the engine starting, generating atorque-control torque command for the electric machine that defines amagnitude that is based on driver-demanded torque.
 20. The method ofclaim 18 further comprising, in response to the bypass clutch beingclosed, generating a torque-control torque command for the electricmachine that defines a magnitude that is based on driver-demandedtorque.